This invention relates to an apparatus for inspecting the inner surface of a hole, and more particularly to an apparatus for inspecting the inner surface of a plug inserted in a heat transfer tube used in a heat exchanger, such as a steam generator, for example.
FIG. 9 schematically shows a typical conventional steam generator. The steam generator 50 shown in this figure is a U-shaped tube type, in which heat transfer tubes 51 are formed in a U shape and their ends are secured in a tube plate 6. These heat transfer tubes serve for heat exchange between a primary-side fluid 52 and a secondary-side fluid 53. The primary-side fluid 52 passes through the heat transfer tubes 51 via an inlet nozzle 54 and a high-temperature-side water chamber 55 and flows out of the steam generator 50 via a low-temperature-side water chamber 56 and an outlet nozzle 57. The secondary-side fluid 53, flowing into the steam generator 50 through an inlet nozzle 58, flows down in an annular vertical space 59, and is diverted upwardly over the top surface of the tube plate 6. While the secondary-side fluid 53 rises around and along the outer surface of the heat transfer tubes 51, it is changed into steam by the heat exchange with the primary-side fluid 52 in the heat transfer tube 51, and finally discharged through an outlet nozzle 60.
These heat transfer tubes 51 define a pressure boundary between the primary-side fluid 52 and the secondary-side fluid 53. Therefore, particularly in a nuclear steam generator, they are important elements for preventing the radioactive primary-side fluid 52 from leaking into the usually nonradioactive secondary-side fluid 53. One of the repairing methods to be employed in the event of leakage through the heat transfer tube 51 is one which is generally called a mechanical plugging method.
FIG. 10 shows a typical conventional plug 8 which can be used for the above-described repair. In this figure, the plug 8 is of a cylindrical shape. This plug 8 has a closed end 8a, while being open at the other end 8b. The inner surface 8c of this plug 8 is in a tapered form such that the inside diameter becomes smaller as it goes from end 8a towards the other end. This plug 8 has a tapered cylindrical member 9 called a wedge core. The wedge core 9 has a through hole 9a in its center, and an internal thread is machined on a surface of the through hole 9a. This wedge core is inserted into the plug before use.
In order to block the heat transfer tube 51 by using the plug 8, the plug 8 is inserted into the inside of the heat transfer tube 51 secured in the tube plate 6 at its both ends, and then the tapered wedge core 9 is pulled down. The internal threads in the through hole 9a are used for threadly engaging a rod or other means for pulling down the wedge core 9.
Thus, the plug 8 is expanded by the wedge action of the wedge core 9, which has been pulled down. By this plug 8, the heat transfer tube 51 is pressed into contact with the wall surface of tube plate 6, and the heat transfer tube 51 is completely blocked, thereby preventing the mixture of the primary-side fluid 52 with the secondary-side fluid 53.
In the case where the heat transfer tube 51 leaks, the plug 8 inserted in the above-described manner constitutes a pressure boundary between the primary-side fluid and the secondary-side fluid. Therefore, the plug 8 is also not allowed to have a fluid leak.
So far, however, there has not been provided any suitable tool for detecting any defects in the upper region L.sub.1 of the plug above the wedge core and the lower region L.sub.2.
The upper region L.sub.1 particularly requires timely or scheduled inspections because the upper portion of the plug 8 is exposed to the secondary-side fluid. So far, however, there has been no tool which can insert a sensor (not shown) into the region L.sub.1 through the small through hole 9a of diameter d which is used for pulling the wedge core 9 outwardly.